In my above identified parent patent application there is described a process for pelletizing sewage sludge to produce a product having pellets of uniform size which can be burned or used as a soil conditioner. In the latter case, uniform size pellets are very important so that the pellets may be used in conventional spreading equipment. The product produced by the parent process is essentially free of fines and free of oversized particles.
In the parent process, the sewage sludge is dewatered to produce a wet sludge cake. Previously pelletized particles and fines are then admixed with the wet sludge cake in a pug mill and the mixture passed through a dryer. The output from the dryer then proceeds to a separator can wherein solids are separated from the off-gas. The solids are then conveyed to a clarifier which separates undersized particles, which are routed to a recycle storage bin, product size pellets which are conveyed to a product storage bin for shipment or sale and oversized particles. The oversized particles are routed to a crusher and then the crushed particles are combined with the fines in a recycle bin for recycling.
The off-gas from the dryer and solid gas separator contains fines and some malodorous gases from the drying process. Typically the off-gas is passed through one or more cyclones to remove most of the fines. The gas is then typically conveyed through a wet scrubber, a packed tower and a chemical tower to remove the undesirable odors before being exhausted to the atmosphere.
It has been known that the dryer off-gas from a sludge incineration process could be used in the incinerator process. For example, in U.S. Pat. No. 4,215,637, a multiple hearth furnace is used for drying and incinerating sludge. Gas from the furnace is conveyed by conduit to an afterburner. Gas from the afterburner then passes through a heat exchanger wherein outside air is heated and the heated outside air is then introduced to the furnace as combustion gas.
Similarly, in U.S. Pat. No. 3,954,069 the drier off-gas after passing through a condenser is readmitted to the sludge incinerator. Also, in U.S. Pat. Nos. 4,153,411 and 4,429,643, the sludge is initially dried and the off-gas, after removal of some of the water therefrom is used in the incinerator.
As noted, the prior art patents all relate to incineration processes and do not describe a process for producing pelletized sludge for use for example as a soil conditioner.